The present invention relates generally to television receivers, and in particular to input circuits for television receivers for separating combined VHF and UHF signals applied to a common input.
In the United States television broadcast stations operate on assigned VHF and UHF frequencies in the range of 50 to 806 MHz. Cable television (CATV) systems, including those now in operation and those proposed for the future, operate within a frequency range which extends between 54 MHz and 806 MHz, excluding only the FM broadcast band (88-108 MHz). Because of the large range of frequencies encountered the usual practice has been to equip television receivers with separate VHF and UHF tuner sections, each designed for optimum performance over its intended frequency range.
In television receivers equipped with separate UHF and VHF tuner sections, it is necessary to apply received UHF television signals to the input of the UHF tuner section, and received VHF signals to the input of the VHF tuner section. For best performance, it is also desirable that the received VHF signals not be applied to the UHF tuner section, and similarly, that the received UHF signals not be applied to the VHF tuner section. In early UHF television broadcasting, a common practice was to employ separate VHF and UHF receiving antennas, each independently connected to the input terminals of respective tuner sections by means of separate transmission lines. Typically, balanced "twin-lead" transmission lines, consisting of two parallel spaced conductors, were used to make such connections.
More recently, television antennas have been developed which include VHF and UHF receiving elements assembled in a single unit. With such antennas, either a single balanced "twin-lead" transmission line having a low-loss dielectric suitable for the VHF and UHF bands, or a single unbalanced coaxial cable, can be used to interconnect the antenna with the television receiver. Coaxial cables are often preferred for their ease of installation, since they can be routed near metallic objects, such as a building plumbing system, with no adverse effect on their signal carrying ability. This is particularly attractive in large apartment or condominium complexes where a common antenna frequently provides television signals to a number of receivers.
Coaxial cable is also the preformed medium for transmitting cable television signals. Accordingly, television receivers intended for receiving CATV as well as broadcast television signals preferably allow direct connection of coaxial cable to the receiver unit.
In using a receiver having separate VHF and UHF inputs to display either broadcast or CATV signals, it is desirable to provide a circuit which separates or "splits" the composite VHF/UHF antenna signal on the twin-lead or coaxial cable into separate isolated VHF and UHF signal components for application to the inputs of the respective tuning sections. Typically, such splitter circuits have been provided as a separate component externally mounted to the television receiver housing adjacent the VHF and UHF input terminals.
The use of externally mounted splitting and matching circuits is in many instances inconvenient and complicates the installation of the television receiver and antenna system. Accordingly, a need has existed for an internal splitter/matching input circuit which provides good isolation between the VHF and UHF components and a single input terminal to which a single VHF-UHF coaxial transmission line can be attached. With such an arrangement, a coaxial line, carrying either a composite VHF/UHF antenna signal or a CATV signal, can be connected directly to a single coaxial connector located on the rear panel of the television receiver. Such an installation is neat, convenient and uncomplicated.
Since CATV operation has been proposed and authorized in the 216-470 MHz band, it is no longer acceptable to allow the VHF section of a signal splitter to begin attenuating signals above 216 MHz, the upper limit of the VHF broadcast band. Splitters compatible with cable systems preferably must allow signals up to 470 MHz to pass to the VHF tuner section substantially unattenuated, while providing sharp attenuation of frequencies above that limit. Accordingly, such compatibility with cable systems should be considered in designing a signal splitting circuit for use with modern television receivers.
In one previous single input connector arrangement, electronic switches were provided for connecting a single antenna input to the inputs of either the VHF or UHF tuners in accordance with electronic control signals provided from the television receiver's main tuning control. While the system performed effectively, the use of active elements in the circuit increased the relative expense of the telvision set as well as the possibility of circuit failure. The present application is directed to an improved input circuit which does not require active components, and which is less expensive to manufacture.